If I am unable to easily load my books from calibre to the kindle fire than it isn't worth buying one. I'd rather just spend a little more, get an iPad mini, and use iBooks.

Personally I would suggest a Nexus 7 using the Calibre Companion App (wireless required) and the reader software of your choice. I use Moon+Reader Pro. This Tablet, Apps and calibre plugins provide you with the greatest flexibility in purchasing and reading ebooks.